nixos: Add zram swap module

{ config, lib, pkgs, ... }:

with lib;

let

  cfg = config.zramSwap;

  devices = map (nr: "zram${toString nr}") (range 0 (cfg.numDevices - 1));

  modprobe = "${config.system.sbin.modprobe}/sbin/modprobe";

in

{

  ###### interface

  options = {

    zramSwap = {

      enable = mkOption {

        default = false;

        type = types.bool;

        description = ''

          Enable in-memory compressed swap space provided by the zram kernel

          module. It is recommended to enable only for kernel 3.14 or higher.

        '';

      };

      numDevices = mkOption {

        default = 4;

        type = types.int;

        description = ''

          Number of zram swap devices to create. It should be equal to the

          number of CPU cores your system has.

        '';

      };

      memoryPercent = mkOption {

        default = 50;

        type = types.int;

        description = ''

          Maximum amount of memory that can be used by the zram swap devices

          (as a percentage of your total memory). Defaults to 1/2 of your total

          RAM.

        '';

      };

      priority = mkOption {

        default = 5;

        type = types.int;

        description = ''

          Priority of the zram swap devices. It should be a number higher than

          the priority of your disk-based swap devices (so that the system will

          fill the zram swap devices before falling back to disk swap).

        '';

      };

    };

  };

  config = mkIf cfg.enable {

    system.requiredKernelConfig = with config.lib.kernelConfig; [

      (isModule "ZRAM")

    ];

    # Disabling this for the moment, as it would create and mkswap devices twice,

    # once in stage 2 boot, and again when the zram-reloader service starts.

    # boot.kernelModules = [ "zram" ];

    boot.extraModprobeConfig = ''

      options zram num_devices=${toString cfg.numDevices}

    '';

    services.udev.extraRules = ''

      KERNEL=="zram[0-9]*", ENV{SYSTEMD_WANTS}="zram-init-%k.service", TAG+="systemd"

    '';

    systemd.services =

      let

        createZramInitService = dev:

          nameValuePair "zram-init-${dev}" {

            description = "Init swap on zram-based device ${dev}";

            bindsTo = [ "dev-${dev}.swap" ];

            after = [ "dev-${dev}.device" "zram-reloader.service" ];

            requires = [ "dev-${dev}.device" "zram-reloader.service" ];

            before = [ "dev-${dev}.swap" ];

            requiredBy = [ "dev-${dev}.swap" ];

            serviceConfig = {

              Type = "oneshot";

              RemainAfterExit = true;

              ExecStop = "${pkgs.stdenv.shell} -c 'echo 1 > /sys/class/block/${dev}/reset'";

            };

            script = ''

              set -u

              set -o pipefail

              PATH=${pkgs.procps}/bin:${pkgs.gnugrep}/bin:${pkgs.gnused}/bin

              # Calculate memory to use for zram

              totalmem=$(free | grep -e "^Mem:" | sed -e 's/^Mem: *//' -e 's/  *.*//')

              mem=$(((totalmem * ${toString cfg.memoryPercent} / 100 / ${toString cfg.numDevices}) * 1024))

              echo $mem > /sys/class/block/${dev}/disksize

              ${pkgs.utillinux}/sbin/mkswap /dev/${dev}

            '';

            restartIfChanged = false;

          };

      in listToAttrs ((map createZramInitService devices) ++ [(nameValuePair "zram-reloader"

        {

          description = "Reload zram kernel module when number of devices changes";

          serviceConfig = {

            Type = "oneshot";

            RemainAfterExit = true;

            ExecStartPre = "${modprobe} -r zram";

            ExecStart = "${modprobe} zram";

            ExecStop = "${modprobe} -r zram";

          };

          restartTriggers = [ cfg.numDevices ];

          restartIfChanged = true;

        })]);

    swapDevices =

      let

        useZramSwap = dev:

          {

            device = "/dev/${dev}";

            priority = cfg.priority;

          };

      in map useZramSwap devices;

  };

}

  ./config/timezone.nix

  ./config/unix-odbc-drivers.nix

  ./config/users-groups.nix

  ./config/zram.nix

  ./hardware/all-firmware.nix

  ./hardware/cpu/intel-microcode.nix

  ./hardware/cpu/amd-microcode.nix